A so-called local area network (LAN) is a spatially limited network in which various network components are coupled to one another. The network components may be one or a plurality of servers and work stations, so-called nodes, that are connected to one another via communication lines in the form of coaxial, optical-fiber or twisted-pair cables. Communication between the network components within the LANs is carried out on the basis of network protocols.
In this context, the Ethernet protocol is the most widespread standard of network protocols for an LAN. Ethernet telegrams may comprise a user-data block with a length of 1500 bytes and allow for telegram-transmission rates between the individual network components within the LAN of several Gigabytes per second wherein the telegrams are transmitted in a package-oriented manner. The Ethernet protocol is also used in industrial production plants in the form of the so-called “Industrial Ethernet”. In automation technology, the Ethernet protocol may e.g. serve to control data exchange in manufacturing, building and process automation with regard to control tasks and to guarantee the real-time capability of the system.
The automation-communication networks used in automation technology generally correspond to so-called “field-bus systems”. Field-bus systems are bus systems in which distributed devices of a machine periphery such as input and/or output modules, drives and operational terminals are connected to control units via the bus system. A shared transmission channel (e.g. as a field bus or radio link) is provided for exchanging data. If a plurality of subscribers send telegrams at the same time, a possibility has to be provided for the subscribers of sharing the transmission channel in mutual consent. One such possibility is using the “master-slave principle”.
Usually, the control units at the field bus are the active bus subscribers, in the following referred to as “master subscribers”. The master subscribers are provided with the access rights of the field-bus system and determine the data flow on the field bus in the form of telegrams. The passive field-bus subscribers, in the following referred to as “slave subscribers”, mostly correspond to the peripheral machine devices. The slave subscribers do not have access rights and may only acknowledge received data or, respectively, telegrams or transmit data or, respectively, telegrams upon request by the master subscriber.
Distribution nodes that are also referred as “switches” are frequently used in automation-communication networks in order to connect the individual network segments having network components to one another and to ensure that the data or, respectively, telegrams within the network segments quickly reach their destination. The telegrams to be transmitted in the automation-communication network may be cyclic real-time telegrams that are relevant for controlling the automation system and comprise cyclic input/output data of the subscribers, and acyclic non-real-time telegrams that e.g. comprise parameters or status data of the subscribers. Cyclic real-time telegrams are in general scheduled telegrams having a fixed transmission time for being transmitted by the distribution nodes, whereas acyclic non-real-time telegrams frequently occur in the telegram flow in an unscheduled manner and do not have a fixed transmission time for being transmitted.
In order to forward the scheduled cyclic real-time telegrams, a switching table, a so-called “routing list”, is deposited in the distribution node that e.g. comprises a telegram identification, e.g. in the form of a MAC address, an input/output interface and a transmission time. In contrast to the described “routing list” for the scheduled cyclic telegrams, the “routing list” for the unscheduled acyclic non-real-time telegrams does not comprise a transmission time as the unscheduled acyclic telegrams do not have a transmission time.
In order to ensure that the forwarding of non-real-time telegrams that will in the following be referred to as unscheduled telegrams will not delay the forwarding of real-time telegrams that will in the following be referred to as scheduled telegrams, a data-transmission cycle for the telegrams is divided up into three time sections in the distribution node of U.S. Pat. No. 8,179,923 B2.
In a first time section, the so-called “cyclic time section”, only scheduled telegrams may be received and forwarded. In the subsequent second time section referred to as “acyclic time section”, the unscheduled telegrams are transmitted. In the third and last time section, the so-called “transitional time section”, acyclic telegrams can further be received, but must not be forwarded.
The transitional time section that at least corresponds to the maximum forwarding time for an unscheduled telegram prevents that a delayed transmission of scheduled telegrams occurs in the subsequent cyclic time section due to the forwarding of an unscheduled acyclic telegram still being in process. The unscheduled acyclic telegrams received in the transitional time section are buffered in the distribution node and transmitted in the next acyclic time section.
In the distribution node, an overload of unscheduled telegrams may easily occur because each distribution node may transmit unscheduled telegrams outside of the cyclic time section, e.g. within the acyclic time section, in an uncontrolled manner. Such overloads particularly occur when the acyclic time section in the telegram or, respectively, data-transmission cycle is considerably shortened compared to the cyclic time section. Due to the operating times within the automation-communication network and in order to achieve down times that are as short as possible during telegram or, respectively, data transmission, the scheduled telegrams are frequently not transmitted directly one after the other in the cyclic time section, but with intermediate time gaps. As a result, the cyclic time section extends. In such a case, there is a danger of the receiving buffer for the unscheduled telegrams in the distribution node overflowing if too many unscheduled telegrams are received in the transitional time period and in the acyclic time section.
Moreover, according to the procedure described in U.S. Pat. No. 8,179,923 B2, the automation-communication network comprising the distribution node has to be previously set up during a configuration phase. If changes occur in the scheduled telegrams that are transmitted in the cyclic time section, a new configuration has to be generated and stored in the distribution node. If a plurality of distribution nodes is used in the automation-communication network, the new configuration has to be additionally transmitted to the distribution nodes. This means that such a change in the configuration may take up several data-transmission cycles. In U.S. Pat. No. 8,179,923 B2, the unscheduled telegrams may only be transmitted in the acyclic phase. An unscheduled telegram is then fully transmitted in the time slot in which no scheduled telegrams are transmitted. However, the disadvantage of the method of U.S. Pat. No. 8,179,923 B2 is that the unscheduled telegram has to have a length that at most corresponds to the remaining time slot until the next scheduled telegram is transmitted so that the acyclic telegram can be transmitted within this free time slot.